1. Field of the Invention
The present invention relates to a fixing apparatus applied to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an apparatus for coating a releasing agent to a fixing member.
2. Related Background Art
FIG. 9 shows a fixing apparatus as a background art of the present invention. As shown in FIG. 9, a fixing apparatus 50 comprises a fixing roller 51 as a rotatable fixing rotary member, a pressure roller 52 as a pressurizing rotary member rotated while abutting against the fixing roller 51, a releasing agent coating device 53 as releasing agent coating means, and cleaning devices 54, 55. Incidentally, heaters 56, 57 such as halogen lamps are disposed within the fixing roller 51 and the pressure roller 52, respectively. Further, thermistors 58, 59 are contacted with the fixing roller 51 and the pressure roller 52 so that surface temperatures of the fixing roller 51 and the pressure roller 52 are adjusted by controlling voltages to the heaters 56, 57 by means of a temperature adjusting circuit.
The releasing agent coating device 53 comprises a releasing agent containing portion 53a for containing releasing agent such as silicone oil, pick-up rollers 53b, 53c for picking up the releasing agent from the releasing agent containing portion 53a, a coating roller 53d for coating the picked-up releasing agent to the fixing roller 51, and regulating blade 53e formed from an elastic material such as fluororubber and adapted to regulate an amount of the releasing agent on the coating roller to a given amount. Particularly in order to realize uniform oil coating on the fixing roller 51, the releasing agent coating device 53 is disposed at a downstream side of the thermistor 58 in a rotational direction of the fixing roller 51.
When a recording paper P is conveyed, the fixing roller 51 and the pressure roller 52 are rotated and the silicone oil is coated on the surface of the fixing roller 51 as the releasing agent. While the recording paper P is being passed between the fixing roller 51 and the pressure roller 52, the recording paper is pressurized and heated from both sides with substantially uniform pressure and temperature, with the result that an unfixed toner image is fixed to the recording paper P, thereby forming a permanent image. Thereafter, the recording paper P to which the toner image was fixed is separated from the pressure roller 52 by a lower separation claw 68 and is discharged out of the apparatus.
Generally, in the thermal fixing apparatus for a copying machine, after a power supply is turned ON, the copying machine is waiting for a predetermined time period until the fixing roller reaches a predetermined temperature, and, after the predetermined temperature is reached, a copy permitting condition is obtained. However, a fixing ability is differentiated between a condition (referred to as "first run in the morning condition" hereinafter) immediately after the copy permitting condition is reached and a condition (referred to as "left condition" hereinafter) after a predetermined time period is elapsed from the copy permitting condition as a stand-by condition. The reason is that not only the surface temperature of the fixing roller but also the temperature of the entire fixing apparatus and particularly the temperature of the releasing agent to be coated on the surface of the fixing roller are differentiated between the first run in the morning condition and the left condition. Namely, in the left condition, since the heat from the fixing roller is transferred to the releasing agent through the coating roller and the pick-up rollers, the temperature of the releasing agent is increased. In this condition, even when the releasing agent is coated on the fixing roller, the reduction of the temperature of the fixing roller is small, and, since the releasing agent between the fixing roller and toner has high temperature, the fixing ability is not improved.
On the other hand, in the first run in the morning condition, the temperature of the releasing agent is low, and, when the low temperature releasing agent is coated on the fixing roller, the temperature of the fixing roller is decreased rapidly, and, since there is low temperature releasing agent between the fixing roller and the toner, the fixing ability is worsened. This will be more noticeable under a low temperature environment.
In order to solve such problems, an arrangement for heating the releasing agent from outside of the releasing agent containing portion has been proposed.
However, the heat from outside of the releasing agent containing portion is insufficient to heat the releasing agent quickly and adequately.
Japanese Patent Application Laid-Open No. 58-200266 discloses a technique in which a heater is disposed within oil in order to maintain fluidity of the oil and to aid an operation of a bimetal.
However, if the heater is disposed within the oil, since conductive portions may be short-circuited, a more safety technique is desired.